Most modern engines are direct injection engines, such that each combustion cylinder of the engine includes a dedicated fuel injector configured to inject fuel directly into a combustion chamber. While direct injection engines represent an improvement in engine technology over past designs (e.g., carburetors) with regard to increased engine efficiency and reduced emissions, direct injection engines can produce relatively high levels of certain undesired emissions.
Engine emissions can include soot, which results from combustion of a fuel-rich and oxygen-lean fuel mixture. Soot comprises small carbon particles created by the fuel-rich regions of diffusion flames commonly created in a combustion chamber of an engine, which may be operating at medium to high load. Soot is an environmental hazard, an emission regulated by the Environmental Protection Agency (EPA) in the United States of America, and the second most important climate-forcing species (carbon dioxide being the most important). Currently, soot is removed from the exhaust of diesel engines by large and expensive particulate filters in the exhaust system. Other post-combustion treatments may also have to be utilized, such as NOx selective catalytic reduction, a NOx trap, oxidation catalyst, etc. These after-treatment systems have to be maintained to enable continued and effective reduction of soot/particulates and other undesired emissions, and accordingly add further cost to a combustion system both in terms of initial equipment cost and subsequent maintenance.
A focus of combustion technologies is burning fuel in leaner mixtures, because such mixtures tend to produce less soot, NOx, and potentially other regulated emissions such as hydrocarbons (HC) and carbon monoxide (CO). One such combustion strategy is Leaner Lifted-Flame Combustion (LLFC). LLFC is a combustion strategy that does not produce soot because combustion occurs at equivalence ratios less than or equal to approximately two. The equivalence ratio is the actual ratio of fuel to oxidizer divided by the stoichiometric ratio of fuel to oxidizer. LLFC can be achieved by enhanced local mixing of fuel with the charge-gas (i.e., air with or without additional gas-phase compounds) in the combustion chamber.